Quantum key distribution (QKD) refers to communication methods that use quantum mechanical principles to guarantee secure communication between two parties. In QKD, a (random) secret key is shared between the two parties, where the key is known only by the two parties to the communication. The key is used to encrypt and decrypt messages. Security of communication between the two parties is assured as a result of the quantum uncertainty principle. If an eavesdropper on a communications channel measures data being transmitted, anomalies are introduced in the data that are then passed down the communications channel and received by a receiver that is party to the secure communication. The receiver can detect the eavesdropper by observing the presence of these anomalies, and can either cease the communication or discard compromised bit values of the shared key.
Discrete variable QKD (DV-QKD) systems modulate and analyze properties of single photons in optical signals to encode and decode data for QKD. DV-QKD systems can encode data on polarization states of single photons, where a polarization state of a photon can represent a logic “1” or a logic “0”. A DV-QKD protocol, BB84, defines a method for performing DV-QKD in which two measurement basis sets are used by a transmitter to transmit information, wherein each basis set defines two different photon polarization states corresponding to the two logic states, for a total of four possible polarization values. A receiver, not knowing which basis set a photon was transmitted in, measures the polarization state of the photon in a randomly-chosen basis. The transmitter and the receiver then compare chosen bases for each measurement to securely determine which of a plurality of communicated bits will make up a shared encryption key.